Liquid propellants



3,070,473 Patented Dec. 25, 1962 3,070,473 lLIQUiD ERGFELLANTS Ronald A. Henry, (China Lake, Calif., and Charles W. Wyandotte, Mich, assignors to the United States of America as represented by the Eiecretary oi the Navy No Drawing. Fit ed Nov. 1, i957, Ser. No. 694,d56 6 Ciainss. (Cl. 149-89} (Granted under Titie 35', US. Code (i952), sec. 265) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to new liquid monopropellent formulations derived from nitroalkanes.

Two of the disadvantages connected with the use of certain monopropellants, such as nitromethane, are that they are difiicult to ignite and their burning rates are not particularly high. Nitromethane, for example, requires a high chamber pressure for its ignition and smooth, steady burning. High burning rates in the region of useful rocket pressures are desirable in the interest of increased thrust.

it is possible to obtain better ignitability and smoothness of burning by using hypergolic bipropellent systems, such as, hydrazine-nitric acid systems, but such systems in a rocket are mechanically more complicated than that required for a simple monopropellant. The smooth ignition and burning of nitromethane at low pressures (300- 800 p.s.i.a.) can be accomplished by injecting small amounts of oxygen into the combustion chamber but this method is essentially a bipropellent system and posesses the mechanical problems associated with such systems. For example, all the difiiculties inherent in handling compressed or liquid oxygen are retained.

It is therefore an object of this invention to provide a iethod for improving the ignitability and burning rate of liquid monopropellants without prohibitively altering their stability or calorific value.

It is another object of this invention to provide liquid monopropellent formulations ignitable at low chamber pressures and having increased burning rates.

It has been found that the above objects are accomplished by the addition to the monopropellant of up to about ten percent of an ultraviolet light absorber. The light absorbers found suitable are certain hydroxy-substituted benzophenones. While the explanation of the surprising etiect produced by these additives is not definite ly known, it is believed to be due to an increase in the amount of sub-surface reaction or decomposition produced by them.

The invention is illustrated by the following examples but is not limited thereby.

Example I A mixture of 2,2'-dihydroxy-4,4-dimethoxybenzophenone and 2,2,4,4'-tetrahydroxybenzophenone was dissolved in nitromethane at a concentration level of about two percent by weight. Strands were made from the formulation and the strand burning rate determined in 4 mm. inside diameter glass tubes over the pressure range 750-1900 p.s.i.g. by the procedure or" Tait, Williams and Whittaker, iournal of American Rocket Society, volume 85, page 83 (1951). This method was used in all examples set forth in this specification. The burning rate was about ten percent faster than that for nitromethane without the additive. More significantly, the pressure at which ignition and smooth burning occurred was lowered from about 1250-1300 p.s.i.g. for nitromethane alone to 775- 800 p.s.i.g. for the mixture. The experiment was repeated with both five percent and ten percent by weight of the additive mixture with substantially the same results. The

limit of solubility of the additive mixture in nitromethane is sli htly over ten percent. These results indicate that only small quantities of the ultraviolet light absorber are needed to effect very profound changes in the properties of the propellant.

Example II 2,4-dihydroxybenzophenone was dissolved in nitromethane at a concentration level of two percent by weight; strands were made from the composition and the strand burning rate determined. An increase in burning rate of 10-20 percent resulted throughout the range of 13002100 p.s.i.g. Smooth burning was attained at a pressure of 700 p.s.i.g. Ten percent of 2,4-dihydroxybenzophenone added to nitromcthane did not further lower this pressure but it did increase the burning rate another 5l0 percent.

In order to test the effectiveness of the ultraviolet lights absorbers on nitric acid systems it was first necessary to nitrate them as they cannot be added to a nitric acid system without reacting and causing some reduction in nitric acid concentration. Although the ultraviolet light absorbers would be operative in the unnitrated form it is preferable to nitrate them before use. Two polynitro 2,4-dihydroxybenzophenones were synthesized as described below.

3,5-dinitro-2,4-dihydroxybenzophenone was prepared as follows: Three grams of 2,4-dihydroxybenzophenone, 60 ml. of 4 N nitric acid solution, and a trace of sodium nitrite were mixed and allowed to stand at room temperature for eight days. The mixture was shaken occasionally. Although the 2,4-dihydroxybenzophenone did not dissolve, it gradually changed color from white to pink. The solid was removed by filtration, washed with cold water, and then extracted with three 45 ml. portions of five percent aqueous sodium bicarbonate. The unnitrated starting material did not dissolve; the aqueous filtrate was acidified and cooled to 5 C. The product was filtered, washed with cold water and dried. The yield was 2.55 g.; MP. 180185 C., wet at 175 C. Two recrystallizations from ethanol gave rosettes of pale yellow needles, MP. 187-188" C.

Analysis.Calcd. for C l-i O N t C, 51.32; H, 2.65; N, 9.21. Found: C, 51.41; H, 3.39; N, 8.55.

The same product (3.0 g.) was formed when 6.0 g. of 2,4-dihydroxybenzophenone was stirred for 2.5 hours with ml. of 4 N nitric acid at 5560 C.

The residues obtained by evaporating the ethanolic recrystallization liquors melted at 170190 C. indicating a mixture. The nitrogen analysis (9.09%) was still consistent, however, with a dinitro-Z,4-dihydroxybenzophenone.

3,3,5 trinitro 2,4 dihydroxybenzophenone was prepared as follows: 2,4-dihydroxyzenzophenone (10.7 g.; 0.05 mole) was added in small portions to 40 ml. of concentrated sulfuric acid (d.=l.84); the temperature was maintained at 10-15 C. by good stirring and cooling. Then 13 ml. of concentrated nitric acid (d=l.42) was added dropwise during two hours while the temperature was held at 510 C. Near the end of the addition, solid began to separate. The nitration mixture then stood overnight at room temperature after which it was heated at 50 C. on the steam bath for 30 minutes. The recooled mixture was poured over 300 g. of crushed ice. The white solid which separated was removed by filtration, washed twice with cold water, and air dried; 17.6 g., M.P. -170" C. This impure product was fractionally crystallized from 95% ethanol. The first crop (5.8 g.) consisted of coarse, yellow needles which melted at 178-180 C.

Analysis.--Calcd. for a dinitro derivative C H O N C, 51.32; H, 2.65; N, 9.21. Calcd. for a trinitro derivative, C13H709N31 C, 44.71; H, 2.02; N, 12.03. Found: C, 45.69, 45.55; H. 2.21, 2.26; N, 12.12, 11.68.

These analyses indicate that the product is the trinitro- 2,4-dihydroxybenzo-phenone contaminated with 12-14% of the dinitro compound.

Two other crops were recovered from the fractional crystallization: (1) 3.4 g., M.P. l68-173 C., N (found)=11.20%; (2) 2.3 g., M.P. 164-1'75 C., N (found)=10.77%.

Example ll! 3,5-dinitro-2,4 dihydroxybenzophenone was dissolved in nitromethane at a concentration level of about two percent and the strand burning rate determined. The burning rate was increased 15-30 percent. Smooth burning was attained at 800 p.s.i.g. Substantially the same results were obtained using 3,3',5-trinitro-2,4-dihydroxybenzophenone as the additive.

Example IV Example V 3,3,5 trinitro 2,4 dihydroxybenzophenone was dissolved in the nitric acid-2-nitropropane system referred to above at a concentration level of two percent by weight and the strand burning rate determined. The results obtained were substantially the same as those obtained in Example IV.

A minor advantage of the polynitro-2,4-dihydroxybenzophenones as additives, over the unnitrated compounds is the fact that they have a less negative oxygen balance and, therefore, do not require as much oxidizer for combustion. This advantage is especially important with nitromethane and other monopropellants which already have a deficiency of oxygen for combustion to carbon monoxide and water. Tests indicated that the optimum amount of ultraviolet light absorber is between about one and about ten percent by weight.

Ultraviolet light absorbers other than those used in the that it provides a simple method for increasing the burn ing rate and ease of ignition of liquid monopropellants derived from nitroalkanes. The additive is simply dissolved in the propellant or propellent system in the amounts disclosed and the propellant used in liquid propellent rockets of the type now well known in the art.

What is claimed is:

1. Liquid propellent compositions consisting essentially of a material from the class consisting of nitroalkanes and mixtures of nitric acid and nitroalkanes, said compositions being catalyzed by the addition thereto of not more than ten percent by weight of a substituted benzophenone having the formula:

u! OH n wherein R is a member from the group consisting of alkoxy and hydroxyl, R is a member from the group consisting of hydrogen, hydroxyl and alkoxy, R is a member from the group consisting of hydrogen and hydroxyl, and R is hydrogen or nitro.

2. The composition of claim 1 in which the substituted benz-ophenone is 2,4-dihydroxyzenzophenone.

3. The composition of claim 1 in which the benzophenone is 2,2'-dihydroxy-4,4'-dimethoxybenzophenone.

4. The composition of claim 1 in which the benzophenone is 2,2',4,4'-tetrahydroxybenzophenone. V

5. The composition of claim 1 in which the benzephenone is 3,5-dinitro-2,4-dihydroxybenzophenone.

6. The composition of claim 1 in which the benzophenone is 3,3',S-trinitro-2,4-dihydroxybenzophenone.

References Cited in the file of this patent UNITED STATES PATENTS 

1.LIQUID PROPELLENT COMPOSITIONS CONSISTING ESSENTIALLY OF AMATERIAL FROM THE CLASS CONSISTING OF NITROALKANES AND MIXTURES OF NITRIC ACID AND NITROALKANES, SAID COMPOSITIONS BEING CATALYZED BY THE ADDITION THERETO OF NOT MORE THAN TEN PERCENT BY WEIGHT OF A SUBSTITUTED BENZOPHENONE HAVING THE FORMULA: 